1. Field of the Invention
The present invention relates to an imaging device and a manufacturing method thereof. More specifically, the present invention relates to a CMOS imaging device capable of performing charge transfer efficiently and to a manufacturing method thereof.
2. Description of the Related Art
In recent years, mobile devices such as cellular phones have kept becoming more and more multifunctional. Among other things, a camera phone capable of easily taking a moving picture and a static image has been widespread, due to reflection of user's interests. It is necessary for such a mobile device to save power so as to be durable for long-time use. Moreover, as for a camera (hereinafter, referred to as an imaging element) mounted on the cellular phone, it is preferable to adopt a CMOS image sensor, which is more suitable for lowering power consumption in comparison with a CCD. The CMOS image sensor has an advantage of being inexpensive because it can be fabricated by a CMOS process required for forming a peripheral circuit, as well as an advantage of consuming less power.
There are some types in a structure of the CMOS image sensor. One of the structure types is disclosed in FIG. 1 of Patent Document 1. According to the structure, electrons generated in a photodiode pass under a transfer gate and are transferred to a floating diffusion region, charges in the floating diffusion region are converted into a voltage in a drive transistor, and the voltage is outputted as a signal voltage to the outside.
Need exists, in the CMOS image sensor, not to lower its charge transfer efficiency, even if the voltage thereof comes to be lowered. In consideration of this point, in Patent Document 1, the transfer gate is extended onto the floating diffusion region as shown in FIG. 6 of the document, and thus a capacitive coupling between the transfer gate and the floating diffusion region is increased. According to this structure, a potential of the floating diffusion region is raised to a positive potential of the transfer gate due to the capacitive coupling described above when a channel of the transfer gate is turned on, resulting in efficient transfer of the electrons from the channel of the transfer gate to the floating diffusion region.
On the other hand, in Patent Document 2, conductivity of a surface layer of a silicon substrate, which serves as the channel of the transfer gate, is set at the N type as shown in FIG. 1 of the document. Thus, the channel and the floating diffusion region are set at the same conductive type, and the charges are allowed to be transferred smoothly from the transfer gate to the floating diffusion region.
Besides the above, technologies relating to the present invention are also disclosed in Patent documents 3 to 5.
(Patent Document 1)
    Japanese Patent Laid-Open No. 2003-101006(Patent Document 2)    Japanese Patent Laid-Open No. 2003-115580(Patent Document 3)    Japanese Patent Laid-Open No. Hei 8(1996)-335688(Patent Document 4)    Japanese Patent Laid-Open No. 2000-152083(Patent Document 5)    Japanese Patent Laid-Open No. 2002-110957
However, Patent Document 1 described above does not go beyond disclosing the structure of extending the transfer gate onto the floating diffusion region, and does not discover a method for realizing such a structure.
Meanwhile, in Patent Document 2, the conductivity of the entire silicon substrate under the transfer gate is set at the N type. Therefore, the channel of the transfer gate becomes prone to be turned on, and the electrons accumulated in the photodiode become prone to overflow to the floating diffusion region through the transfer gate. In such a case, an amount of electrons capable of being accumulated in the photodiode is reduced, and a value of the signal voltage obtained by converting the electrons to a voltage is also reduced. Accordingly, a ratio (S/N ratio) of the signal voltage value and a noise voltage value becomes reduced, causing a possibility that noise of the imaging device is increased.